Diazo reproduction processes based on light-sensitive diazonium compounds and their ability to form azo dyes with a coupling component under proper pH conditions are generally well known. The various techniques for carrying out the development include the moist or semi-moist process, the dry or ammonia process, and the thermal process. The diazo copying materials adapted for the moist process are referred to as one-component materials since they contain only the light-sensitive diazo compounds, and the coupling component is provided in a separate buffered developing solution which is applied at the time of development. Diazotype materials developed in an atmosphere of water vapor and ammonia gas are generally referred to as two-component materials since they contain both the diazonium salt and the coupling component stabilized against premature coupling in an acidic medium. In the thermal systems heat is employed to release the alkaline agents which are coated on the copy material along with the diazonium salt and the coupler. Each of these systems provides a successful copying process but not without certain disadvantages that limit their utility which would otherwise permit diazo copying to enjoy more widespread application as a reproduction medium.
Two-component materials have found wide use in the reproduction of engineering drawings. The process of developing with ammonia gas and water vapor is fast and essentially dry. Ammonia vapor readily penetrates into the exposed copy sheet surface and permeates the light-sensitive coating containing acid stabilizers, shifting the pH of the coating to the alkaline side. Any excess of ammonia enhances the rate of the reaction and volatilizes from the copy sheet without leaving a residue. One of the major objections to the use of ammonia in developing diazotype materials is the strong odor of ammonia in the copying area as well as the tendency for the odor to linger on the copies themselves. Installation of ammonia reproduction machines requires venting of the equipment to the outdoors to remove the toxic vapors. This limits the installation of the equipment to sites where the venting may be conveniently accomplished. The equipment is further complicated by the need for a gas-developing chamber to contain the ammonia gas and water vapor and the other appurtenances required for feeding the developing ingredients to the chamber. The equipment, understandably, is not well suited for office installations.
The one-component systems have effectively eliminated the odor problem by applying liquid developers directly to the copy sheet. Reproduction equipment for use with the one-component type papers requires heating elements to dry the copy sheet after development.
Thermal systems, while offering the advantages of dry copying without the presence of undesirable odors, suffer in that the copying materials themselves may lack stability. The machines are objectionable because of the amount of heat given off into the surrounding work area.
Attempts have been made to simplify the processing of diazotype materials, particularly the two-component types, by using alkaline liquids applied directly to the latent image bearing surface. These prior attempts to eliminate the complexities of gaseous development have been unsuccessful for a number of reasons.
By the procedure of the Parker U.S. Pat. No. 3,578,452 it has been possible to use a liquid organic amine as the developer for two-component diazotype materials and produce dry copies directly from the developing step. Development is accomplished by supplying controlled amounts of a concentrated developer solution, preferably comprising an aliphatic amine in a solvent, to a developing region and then passing an exposed diazotype paper through the developing region. At the developing region the controlled small amount of developer liquid is applied directly to the surface bearing the latent diazo image to effectively develop the azo image and produce prints which emerge dry, odor-free and ready for use.
Within the developing region the limited quantity of developer liquid is applied under pressure, thus being spread out uniformly over its entire surface in a layer having a thickness of about 1 micron, which is sufficient to cause rapid image development. The azo dye image is found to develop evenly and uniformly everywhere across the copy area, indicating the effectiveness of the method. A visible, readable image begins to form instantaneously as the copy emerges from the developing region and rapidly increases in density, attaining its maximum image density well within one minute and usually within 15 seconds. Understandably, the rate at which maximum density is reached will depend in part on the couplers and diazonium compounds used in making the copy sheets.
Although the color of the azo dye image which is obtained in any given instance depends primarily on the coupling components and the diazonium compounds which are employed, coupling components are often generally described as being couplers of a given color -- the color being the color of the dye which is usually obtained when the particular coupler in question couples with a diazonium compound. For example, couplers such as monohydric phenols, catechols, catechol derivatives, resorcinols, resorcinol derivatives, diketones, acetoacetic acid derivatives, acetonitriles, cyanacetylamides and the like, usually result in yellow, orange, sepia, brown, red or maroon azo dyes. Thus, couplers from such classes of materials are conveniently referred to as yellow, orange, sepia, brown, red, or maroon couplers. On the other hand, couplers such as naphthoic acid derivatives, dioxynaphthalene derivatives, pyronones, hydroxypyronones, and the like, usually result in blue or violet azo dyes, and thus are conveniently referred to as blue or violet couplers.
One group of highly useful coupling components are the yellow couplers, since the dyes obtained from these couplers usually have actinic adsorption characteristics which permit their use as the sole coupler in a diazo composition which is employed to prepare diazotype "masters" or intermediates, and since couplers from this group can often be employed as shading components when used in conjunction with another coupler or couplers. As indicated above, compounds containing active methylene groups, compounds such as acetonitriles, derivatives of acetonitriles, and the like, have been employed as yellow couplers in diazo compositions, cf. for example, U.S. Pat. Nos. 1,989,065; 2,531,004; 2,537,001; 2,537,106 and 3,558,318; yet, a number of these active-methylene types of couplers have exhibited a tendency, when employed in two-component diazo compositions, to precouple with the diazonium compound which is present in said compositions during storage even in the presence of the stabilizers which are usually employed. This tendency to precouple prior to exposure and development has limited the use of these materials somewhat, since even a slight amount of precoupling can result in the formation of an azo dye in those areas of the diazotype material which are the background or "cleared" areas of the diazotype print. In addition to this tendency to precouple, a number of these prior art, active-methylene types of couplers also result, upon coupling, in dyes which have an undesirable reddish blue.
Well known among the yellow couplers are such compounds as cyanoacetmorphilide and cyanoacetanilide. The former is very slow coupling whereas the latter is extremely fast but results in materials with a greatly reduced shelf life. Ideally, a coupler should be reasonably faster than the known slow reacting yellow coupler without suffering the detriment of poor shelf life.